JIS G4305:2015 limits the maximum phosphorus content in SUS304 stainless steel round bars to 0.045%. Phosphorus is a residual element from steelmaking, but high levels have two key drawbacks:
Brittleness: Phosphorus segregates at grain boundaries, weakening intergranular bonds and increasing the risk of brittle fracture, especially at low temperatures.
Corrosion Susceptibility: Excess phosphorus reduces resistance to pitting corrosion in chloride environments (e.g., seawater), as it disrupts the passive chromium oxide layer.To control phosphorus, manufacturers use high-purity raw materials and employ ladle metallurgy processes to remove impurities. Post-production, each batch is tested for chemical composition, with phosphorus levels documented in the material certificate. The 0.045% cap ensures SUS304 maintains its ductility and corrosion resistance, making it suitable for demanding applications like marine hardware or chemical piping.